This invention relates to a production process of polysuccinimide, and more specifically to a production process of polysuccinimide, which comprises conducting polymerization of aspartic acid by using an acidic catalyst.
Polysuccinimide is used as a suitable precursor or intermediate in the production of polyamino acid derivatives such as polyaspartic acid. Polysuccinimide and polyamino acid derivatives, such as polyaspartic acid, are known to have biodegradability, and are extremely useful as polymers compatible with the environment. Further, crosslinked polyaspartic acid salts which are derivatives of polysuccinimide are equipped with both biodegradability and water absorbency, and are extremely useful polymers.
As techniques for producing polysuccinimide by reacting aspartic acid in the presence of an acidic catalyst, there are, for example, the following conventional techniques (i) to (iii).
(i) U.S. Pat. No. 5,142,062
This patent discloses a technique for producing polysuccinimide having a weight average molecular weight of from 100,000 to 200,000 by the following steps: 1) reacting a mixture of aspartic acid and phosphoric acid or the like at 100 to 250xc2x0 C. in a vacuum system lower than 1 bar to produce a solid reaction mixture containing polysuccinimide having a weight average molecular weight of from 10,000 to 100,000, and 2) grinding the solid reaction mixture obtained in the first step into particles, the sizes of which ranged from 0.001 to 2 mm, and then conducting polycondensation under conditions selected from the temperature and pressure ranges in the first stage.
In Example 1, a crude product composed of polysuccinimide and phosphoric acid was produced by mixing aspartic acid (50 g, 0.38 mol) and 85% phosphoric acid (25 g, 0.22 mol in terms of phosphoric acid) [phosphoric acid/aspartic acid molar ratio=0.58] and conducting polymerization at 200xc2x0 C. for 4 hours in a vacuum system. It is disclosed that, when a portion of the crude product was investigated for its molecular weight after washing off phosphoric acid, the crude product was found to have a weight average molecular weight (Mw) of 86,000. In Example 2, polysuccinimide having a weight average molecular weight of 124,000 was obtained by further grinding the crude product of Example 1 into particles, the sizes of which ranged from 001 to 2.0 mm, and conducting polymerization again at 200xc2x0 C. for 4 hours in a vacuum system of 1 mbar.
It is also disclosed from Example 1 and Example 2 that the reaction product changes, in its form, from a fluid into a solid as the reaction proceeds. This technique features that the inclusion of grinding makes it possible to produce polysuccinimide having a high weight average molecular weight. However, to conduct industrial production while suitably dealing with a reaction in which a reaction product undergoes solidification, a special reactor is generally required, thereby making it difficult to design equipment. Especially when the form of the reaction product continuously changes from a fluid into a solid as the reaction proceeds, it is difficult to design a continuous reactor which can meet such changes.
Incidentally, the molar ratio of phosphoric acid to aspartic acid remains constant from the initiation to the end of the reaction in this conventional technique.
(ii) U.S. Pat. No. 5,457,176 (corres. JP 7-216084 A)
This patent discloses a technique for producing an amino acid polymer by heating a mixture of an amino acid and an acidic catalyst. Specifically, it is clearly indicated in column 2, lines 15-16 that an object of this technique is to produce an amino acid polymer the maximum weight average molecular weight of which is 60,000 or lower.
Further, Example 3 discloses an example in which a reaction mixture in the form of wet tacky white powder, which had been obtained by mixing aspartic acid (800 g, 6.01 mol) and 85% orthophosphoric acid (200 g, 1.73 mol in terms of phosphoric acid), was heated in the form of a layer on a stainless steel pan. It is disclosed in Example 3 that, when heated at 240xc2x0 C. for 1 hour, the reaction mixture in the form of the wet tacky white powder changed into a solid mass which was hard on an outer side but was tacky in a central part. Polysuccinimide the weight average molecular weight of which was 15,500 was obtained by grinding that solid mass with a pestle in a mortar and heating the resulting powder further at 240xc2x0 C. for 6 hours.
However, to practice industrial production while suitably dealing with a reaction in which a reaction mixture changes from a tacky form into a solid mass, a special reactor is needed so that the designing of equipment is extremely difficult. In particular, a need arises for a reactor provided with a special mechanism which does not develop a fault or the like in the presence of phosphoric acid at elevated temperatures. It is difficult to design such a reactor.
Among the weight average molecular weights of the polysuccinimide samples disclosed in the Examples, the highest weight average molecular weight is 24,000. This is consistent with the object to produce an amino acid polymer the molecular weight of which is 60,000 or lower.
Incidentally, the molar ratio of phosphoric acid to aspartic acid also remains constant from the initiation to the end of the reaction in this conventional technique.
(iii) U.S. Pat. No. 5,688,903 (corres. JP 8-231710 A)
This patent discloses a process for the production of a polycondensation product of an amino acid or a polypeptide hydrolyzate of the polycondensation product by subjecting the amino acid to bulk polycondensation under heat in the presence of phosphoric acid, phosphorus pentoxide or polyphosphoric acid and optionally, then conducting hydrolysis. This technique features that a fine particulate raw material is prepared with 0.005 to 0.25 mol of the catalyst evenly dispersed therein per molecule of the amino acid and polycondensation is then conducted.
Its Examples disclose illustrative reactions in a vacuum system and illustrative reactions in an atmospheric pressure system. Each of those reactions was conducted after comminuting a raw material, which was composed of a uniform mixture of aspartic acid and phosphoric acid, into a fine particulate raw material in a comminuting machine.
Problems, such as formation of a foam phase during the polymerization and formation of a coherent mass after the polymerization, were overcome by conducting the reaction while using the fine particulate raw material in which the amount of phosphoric acid, phosphorus pentoxide or polyphosphoric acid is controlled in the above-described range. When the viscosity indexes of the polysuccinimide samples as determined in the form of DMF solutions are converted into weight average molecular weights, the weight average molecular weight is, however, about 19,000 in Example 4 and about 28,000 in Example 8. The polysuccinimide samples obtained in the Examples are limited to low molecular weight polysuccinimide samples the weight average molecular weights of which are lower than 30,000. This process is, therefore, not sufficient as a process for the production of high molecular weight polysuccinimide.
In Example 10 (comparative example) in which phosphoric acid was used beyond the above-described range, on the other hand, polysuccinimide the weight average molecular weight was about 76,000 was formed. It is, therefore, appreciated that Example 10 (comparative example) afforded polysuccinimide having a higher weight average molecular weight than the above-described Examples 4 and 9 although formation of a viscous phase and coagulation of the reaction product took place in Example 10. In this conventional technique, the molar ratio of phosphoric acid to aspartic acid also remains constant from the initiation to the end of the reaction.
As mentioned above, polymerizationxe2x80x94in which as in the conventional techniques (i) and (ii), solidification of a reaction product takes place and grinding is requiredxe2x80x94renders the designing of equipment difficult when practice of continuous mass-production is desired. In particular, a need arises for a reactor provided with a special mechanism which does not develop a fault or the like in the presence of phosphoric acid at elevated temperatures. It is difficult to design such a reactor. The conventional technique (iii), on the other hand, can avoid the formation of a viscous phase and the formation of coherent mass of the reaction product owing to the use of a fine particulate reaction raw material in which the amount of phosphoric acid is controlled in the predetermined range. However, the weight average molecular weight of the thus-produced polysuccinimide is low.
As a conclusion, no conventional technique was able to manage to achieve, at the same time, the production polysuccinimide having a high weight average molecular weight and the prevention of formation of a viscous phase, formation of foam and the formation of coherent mass of the reaction product.
Further, there was no process that can produce polysuccinimide having a high weight average molecular weight when solid-state polymerization is conducted in an atmospheric pressure system.
An object of the present invention is to produce polysuccinimide having a high weight average molecular weight.
Another object of the present invention is to provide polysuccinimide by simpler equipment.
A further object of the present invention is to overcome the formation of an extremely viscous phase, the excessive formation of foam, the formation of coherent mass of a reaction production and like problems, all of which arose in the course of production by the conventional techniques, and to produce polysuccinimide suited for continuous mass-production and having a high weight average molecular weight.
A still further object of the present invention is to produce polysuccinimide having a high weight average molecular weight by solid-state polymerization in an atmospheric pressure system.
With a view to achieving the above-described objects, the present inventors have proceeded with an extensive investigation on production processes of high molecular weight polysuccinimide. As a result, it has been found that high molecular weight polysuccinimide having a weight average molecular weight of 40,000 or higher can be produced by a production process in which the following three steps are conducted continuously and/or batchwise:
1) mixing and heating aspartic acid and an acidic catalyst to produce a liquid, low molecular weight polymer mixture;
2) separating the acidic catalyst to appropriate extent from the liquid, low molecular weight polymer mixture, thereby directly changing a polymer-containing phase from a liquid phase into a solid phase to produce a solid, low molecular weight polymer mixture; and
3) conducting solid-state polymerization on the solid, low molecular weight polymer mixture.
Described specifically, the present invention provides a production process of high molecular weight polysuccinimide, which comprises the following steps 1) to 3):
1) polycondensing a mixture, which comprises aspartic acid and an acidic catalyst, a weight concentration C1 of the acidic catalyst based on a total weight of a weight of the aspartic acid and a weight WA1 of the acidic catalyst being indicated by the below-described numerical formula (1), at 80 to 350xc2x0 C. to produce a liquid, low molecular weight polymer mixture comprising low molecular weight polysuccinimide, which has a weight average molecular weight Mw1 indicated by the below-described numerical formula (2), and the acidic catalyst:
20 [wt. %]xe2x89xa6C1xe2x89xa699 [wt. %]xe2x80x83xe2x80x83(1)
1.0xc3x97104xe2x89xa6Mw1xe2x89xa62.0xc3x97105xe2x80x83xe2x80x83(2)
2) from said liquid, low molecular weight polymer mixture produced in step 1, producing a solid, low molecular weight polymer mixture wherein the low molecular weight polymer has a weight average molecular weight Mw2 indicated by the below-described numerical formula (5), and a weight concentration C2 of said acidic catalyst based on a total weight of a weight of low molecular weight polysuccinimide and a weight WA2 of said acidic catalyst is indicated by the below-described numerical formula (3) and said weight WA2 of said acidic catalyst is indicated by the below-described numerical formula (4), and said solid, low molecular weight polymer mixture being free from melting under reaction conditions of the following step 3):
5 [wt. %]xe2x89xa6C2xe2x89xa655 [wt. %]xe2x80x83xe2x80x83(3)
WA2xe2x89xa6WA1xe2x80x83xe2x80x83(4)
1.0xc3x97104xe2x89xa6Mw2xe2x89xa62.0xc3x97105xe2x80x83xe2x80x83(5)
3) subjecting the solid, low molecular weight polymer mixture produced in step 2) to solid-state polymerization at 120 to 350xc2x0 C. to produce a solid, high molecular weight polymer mixture comprising high molecular weight polysuccinimide, which has a weight average molecular weight Mw3 capable of satisfying the below-described numerical formulas (6), (7) and (8) at the same time, and the acidic catalyst:
4.0xc3x97104xe2x89xa6Mw3xe2x89xa61.0xc3x97106xe2x80x83xe2x80x83(6)
Mw1xe2x89xa6Mw3xe2x80x83xe2x80x83(7)
Mw2xe2x89xa6Mw3xe2x80x83xe2x80x83(8)